In communications networks, there may be a challenge to obtain good performance and capacity for a given communications protocol, its parameters and the physical environment in which the communications network is deployed.
For example, for future generations of mobile communications networks, frequency bands at many different carrier frequencies could be needed. For example, low such frequency bands could be needed to achieve sufficient network coverage for terminal devices and higher frequency bands (e.g. at millimeter wavelengths (mmW), i.e. near and above 30 GHz) could be needed to reach required network capacity. In general terms, at high frequencies the propagation properties of the radio channel are more challenging and beamforming both at the network node of the network and at the terminal devices might be required to reach a sufficient link budget.
In a communications network where a transmission and reception point (TRP) at the network side uses narrow beams for transmission, at least one of the narrow transmission beams is assumed to be discovered and monitored for each served terminal device at the user side. This process of discovering and monitoring is referred to as beam management.
In order to perform beam management the network node uses measurements (such as received reference signal power), as obtained and reported by the served terminal devices, on downlink reference signals such as channel state information reference signals (CSI-RS). The beam pair for which the highest received reference signal power was obtained is then used as the active beam pair link. In general terms, a beam pair is defined by a transmission beam at the transmitting end (such as at the TRP) and a corresponding reception beam at the receiving end (such as at the terminal device), where the transmission beam and the reception beam are selected from sets of available candidate beams so as to maximize a quality criterion (such as highest received reference signal power) for transmission from the transmitting end to the receiving end.
In order for a terminal device to find a suitable transmission beam the network node could transmit CSI-RS in different transmission beams on which the terminal device performs measurements and reports back the N≥1 best transmission beams (where the value of N can be configured by the network node).
Furthermore, the CSI-RS transmission in a given transmission beam can be repeated to allow the terminal device to evaluate suitable reception beam as used by the terminal device. This is sometime referred to as a “P3 procedure” or UE RX beam training.
Further, the CSI-RS can be transmitted periodically, semi-persistently or aperiodically (such as when being event triggered) and they can be either shared between multiple terminal devices (users) or be user-specific.
For terminal devices the incoming signals (such as the CSI-RS) might arrive from all different directions, depending on the location and orientation of the terminal device relative the transmitting TRP. Hence it might be beneficial to have an antenna implementation at the terminal device which has the possibility to generate omni-directional-like coverage in addition to the high gain narrow beams. One way to increase the omni-directional coverage at an terminal is to provide the terminal device with multiple antenna arrays, or panels, where the antenna arrays have mutually different pointing directions. In some aspects, a maximum two baseband chains at the terminal device might be used for mmW frequencies. A larger number of baseband chains might generate too much heat for the terminal device, especially for such large bandwidths that are expected at these high frequencies. In some aspects the terminal device might therefore include just one single baseband chain, where one and the same baseband chain can be alternatingly operatively connected, via a switch, to a respective antenna array.
During, for example, a UE RX beam training procedure, there could be many different UE RX beams for the terminal device to evaluate. This will increase overhead signalling. For example, assume that the terminal comprises two antenna arrays and that each antenna arrays is capable of generating 8 beams, then there will be in total 16 different UE RX beams to evaluate during a UE RX beam training procedure. If one orthogonal frequency-division multiplexing (OFDM) symbol is used for each beam, this means that in order to sweep through all beams, 16 OFDM symbols are needed, which requires several slots of overhead signaling.
The same issue is apparent in TRP RX beam a training procedure.
Hence, there is still a need for improved beam training procedure for terminal devices as well as network nodes.